Cathode Material For Li-Ion Batteries

ABSTRACT

The present invention concerns an electrode material of formula Li 2+x Ni u Ti v Nb w O 4  wherein:
     0&lt;x&lt;0.3,   u&gt;0 and w&gt;0,   x+u+v+w=2,   x+2u+4v+5w=6,   the electrode material having a crystal structure of disordered NaCl type. The present invention also concerns the cathode having this material as an electronically active material, and also the lithium-ion battery containing this cathode.

DOMAIN OF THE INVENTION

The invention relates to a material made up of lithium having formulaLi_(2+x)Ni_(u)Ti_(v)Nb_(w)O₄, as well as to the use thereof as a cathodematerial and to the preparation method thereof.

The field of use of the present invention relates to electrical energystorage, and more particularly to lithium-ion batteries.

BACKGROUND

Lithium-ion batteries are particularly well adapted to portableelectronic equipment due to their energy density and to their timestability in terms of charge and discharge cycles.

A lithium-ion battery generally comprises the following assembly:

-   -   a positive electrode (cathode) comprising a lithium-based        material,    -   a negative electrode (anode), generally made up of carbon, for        example, of graphite.

Reversible exchanges of Li⁺ ions between the cathode and the anodeensure its operation. At the cathode level, the materials having thestrongest energy are superstoichiometric layered lithium oxides. Theyenable to reach proper specific capacities (250 mAh/g). However, theyhave many disadvantages, mainly due to the participation of oxygen inthe electrochemical processes, among which:

-   -   a strong irreversibility at the first cycle;    -   a structural instability;    -   a cycling potential loss.

To overcome these problems, it has been envisaged to use materials ofRock-Salt structures (of NaCl type), for example:

-   -   document WO 2009/120156 discloses the Li₂FeTiO₄ material having        a 130-mAh/g capacity at C/20 and 60° C. between 3.9 V and 1.9 V    -   document CN 104269520 discloses the Li₂FeTiO₄ material having a        graphite coating and a 200-mAh/g capacity at C/30 between 5 V        and 1.5 V,    -   document JP 2013-206746 discloses the Li₂NiSi_(1-x)Ti_(x)O₄        material with 0<x<1 having a 120-mAh/g capacity at C/20 between        4 V and 2 V,    -   document WO 2014/73700 describes the        Li₂Ni_((1-x-y))Co_(x)Mn_(y)TiO₄ material with x>0, y>0, having a        230-mAh/g capacity at C/100 between 4.8 V and 2 V.

However, even if Li₂NiTiO₄-type materials having a disordered NaClstructure have a high theoretical capacity (290 mAh/g), based on theoxidation of Ni²⁺ in Ni⁴⁺ only, these materials have a too low ionconductivity, thus limiting the performance of the material.

The Applicant has developed a new lithium-containing material having anion conductivity greater than that of materials of Li₂NiTiO₄ type and atheoretical specific capacity that can reach or exceed 250 mAh/g.

DESCRIPTION OF THE SPECIFICATION

The present invention concerns an electrode material of formulaLi_(2+x)Ni_(u)Ti_(v)Nb_(w)O₄ wherein:

0<x<0.3,

u>0 and w>0,

x+u+v+w=2,

x+2u+4v+5w=6.

In this formula, u and w are different from 0. However, v may be equalto 0; in this case, titanium is integrally substituted with nickel orniobium.

Advantageously, the above formula comprises at least one of thefollowing parameters:

-   -   u may be in the range from 0.9 to 4/3.    -   v may be in the range from 0 to 0.6.    -   w may be in the range from 0.3 to 0.77.

The material of formula Li_(2+x)Ni_(u)Ti_(v)Nb_(w)O₄ is particularlyadapted for a use as a cathode material, particularly in a lithium-ionbattery.

Generally, this material has a crystal structure of disordered NaCltype.

A structure of NaCl type corresponds to two face-centered cubicsub-lattices (atoms distributed at the 8 apexes of a cube and at thecenter of each of the faces of the cube). The two sub-lattices areoffset by half the side length of the mesh.

A disordered structure corresponds to a crystal having its atoms placedregularly in the sites but having an irregular atom distribution.

As already indicated, the material according to the invention has atheoretical specific capacity which can reach or exceed 250 mAh/gwithout involving an electrochemical activity of the oxygen of thelattice due to the Ni²⁺/Ni⁴⁺ couple. Further, the substitution oftitanium with lithium and with a metal (nickel and/or niobium) enablesto improve its ion conductivity and thus its performance.

Generally, lithium-enriched layered oxides enable to obtain strongcapacities since part of the oxygen forming them takes part in theelectrochemical reaction by oxidizing in charge, like metals, whichcreates a structural instability. The material according to theinvention enables to reach 250 mAh/g without for the oxygen of thestructure to oxidize, since the Ni²⁺/Ni⁴⁺ couple provides enoughelectrons.

The material according to the invention may be selected from the groupcomprising: Li_(2.1)NiTi_(0.6)Nb_(0.3)O₄;Li_(2.05)NiTi_(0.8)Nb_(0.15)O₄; and Li_(2.2)NiTi_(0.2)Nb_(0.6)O₄.

As an example of material according to the invention:

-   -   Li_(2.2)NiTi_(0.2)Nb_(0.6)O₄ (x=0.2; u=1; v=0.2; w=0.6) has a        theoretical capacity, without intervention of oxygen, of 263        mAh/g.    -   Li_(2.1)NiTi_(0.6)Nb_(0.3)O₄ (x=0.1; u=1; v=0.6; w=0.3) has a        theoretical capacity, without intervention of oxygen, of 276        mAh/g.

Generally, the theoretical specific mass capacity of the materialaccording to the invention may be in the range from 240 to 285 mAh/g.

The material according to the invention has an ion conductivity greaterthan that of conventional Li₂NiTiO₄-type materials, due to the increaseof the number of percolation paths for lithium. Indeed, lithium beingthe only mobile ion in the structure, it can only diffuse if one of theneighboring sites is also occupied by a lithium ion. The increase in thelithium/metal ratio enables to increase the probability of occurrence ofsuch a configuration and thus to multiply the possible percolationpaths.

The material according to the invention may appear in the form ofparticles and of particle agglomerates.

Advantageously, it is formed of agglomerates of from 1 to 5 micrometersformed of particles. The particles are preferably spherical. Theiraverage diameter is advantageously in the range from 30 to 100nanometers.

The present invention also relates to the method of manufacturing thematerial of formula Li_(2+x)Ni_(u)Ti_(v)Nb_(w)O₄.

It may particularly by a solid, or sol-gel, or hydrothermal, or molten(molten salt) synthesis. Advantageously, it is formed by molten saltsynthesis.

Such synthesis techniques pertain to the general knowledge of thoseskilled in the art and require no specific conditions.

As an example, the synthesis may be performed with molten salts fromlithium, nickel, titanium, and niobium precursors in a mixture ofNaCl/KCl.

The precursors used in this case may in particular be Li₂CO₃,Ni(CH₃COO)₂. 4H₂O, TiO₂, and Nb₂O₅.

The present invention also relates to a cathode where theelectronically-active material is the material, described hereabove, offormula Li_(2+x)Ni_(u)Ti_(v)Nb_(w)O₄.

The present invention also relates to a lithium-ion battery (oraccumulator) comprising this cathode.

Such a lithium-ion battery particularly comprises the assembly of acathode according to the invention, of an electrolyte made up of lithiumsalt, and of an anode, generally made up of carbon (graphite, forexample).

It will be within the abilities of those skilled in the art to preparethis battery by using their general knowledge to implement conventionaltechniques, particularly by deposition of an ink comprising theLi_(2+x)Ni_(u)Ti_(v)Nb_(w)O₄ material.

The invention and the resulting advantages will better appear from thefollowing non-limiting drawings and examples, provided as anillustration of the invention.

DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates diffractograms of the Li_(2+x)NiTi_(1-4x)Nb_(3x)O₄compounds, with x=0.05, 0.10, 0.20.

FIG. 2 corresponds to an enlargement of the diffractograms of theLi_(2+x)NiTi_(1-4x)Nb_(3x)O₄ compounds (x=0.05, 0.10, 0.20) between 30and 50° C.

FIG. 3 illustrates the charge and discharge capacity of the Li₂NiTiO₄material according to the number of cycles.

FIG. 4 illustrates the charge and discharge capacity of theLi_(2.1)NiTi_(0.6)Nb_(0.3)O₄ material according to the number of cycles.

FIG. 5 illustrates the voltage of the Li₂NiTiO₄ material according tothe specific capacity.

FIG. 6 illustrates the voltage of the Li_(2.1)NiTi_(0.6)Nb_(0.3)O₄material according to the specific capacity.

FIG. 7 illustrates the potential according to the charge capacity of theLi_(2+x)NiTi_(1-4x)Nb_(3x)O₄ materials (x=0.05, 0.10, 0.20).

FIG. 8 shows an image obtained by scanning electron microscope of theLi_(2.1)NiTi_(0.6)Nb_(0.3)O₄ material.

EMBODIMENTS OF THE INVENTION

Synthesis

The Li_(2+x)NiTi_(1-4x)Nb_(3x)O₄ (x=0.05, 0.10, 0.20) materials havebeen synthesized by a molten salt process according to the followingprotocol, under air.

The Li₂CO₃, Ni(CH₃COO)₂.4H₂O, TiO₂, and Nb₂O₅ precursors are added instoichiometric proportions to a NaCl/KCl (4 eq mol) eutectic mixture.

After the mixing, the assembly is taken to 350° C. for 2 hours, and thento 670° C. for 3 hours.

After the cooling, the material is washed with distilled water to removethe salt mixture, and then dried at 80° C. under air.

The diffractograms of FIGS. 1 and 2 show the different phasessubstituted with Nb, showing a linear variation of the latticeparameters, which indicates that substitution results in a solidsolution.

Electrochemical Tests

a) Preparation of the Positive Electrode

The active material of formula Li_(2+x)NiTi_(1-4x)Nb_(3x)O₄ (x=0.05,0.10, 0.20) is mixed at 80 wt. % with carbon black (Super P carbon, 10%)and a PVDF binder (polyvinylidene fluoride 10%) dissolved inN-methyl-2-pyrrolidone.

The mixture is then spread on an aluminum foil (100 micrometers) andthen dried at 60° C.

b) Mounting of the Accumulator

The electrode thus formed is introduced into a cell of “button cell”type at format 2032. The negative electrode is made of metal lithium.

Two types of separators are used: a polypropylene film (Celgard® 2400)and a polyolefin film (Viledoe).

The electrolyte used is a compound of ethylene carbonate, of propylenecarbonate, of dimethyl carbonate, and of lithium hexafluorophosphate(LiPF₆) (Electrolyte LP100).

c) Galvanostatic Cycling

At room temperature, a current is imposed to the system to obtain a C/50rate, that is, the extraction/insertion of a lithium ion within 50hours.

d) Results

FIGS. 3 and 4 show that the substitution of titanium with lithium andniobium results in a capacity which is greater (80 mAh/g vs. 91 mAh/g)and more stable during the cycling at C/50 between 4.8 V and 2 V (FIGS.5 and 6).

Such an improvement is imputed to a better ion conductivity of thematerial since the polarization decreases along with the substitution.

FIG. 7 corresponds to the potential according to the charge capacity. Itshows the importance of the substitution of titanium atoms with lithiumand niobium atoms. The larger the substitution rate, the greater thecapacity that can be reached at first charge. For the highestsubstitution rate (x=0.2), 87% of the theoretical capacity are reachedat room temperature and 95% are reached at 55° C.

FIG. 8 corresponds to an image obtained by scanning electron microscopeof the Li_(2.1)NiTi_(0.6)Nb_(0.3)O₄ material. It illustrates thepresence of agglomerates of generally spherical particles.

1. An electrode material of formula Li_(2+x)Ni_(u)Ti_(v)Nb_(w)O₄ wherein: 0<x<0.3, u>0 and w>0, x+u+v+w=2, x+2u+4v+5w=6, the electrode material having a crystal structure of disordered NaCl type.
 2. The electrode material of claim 1, wherein u is in the range from 0.9 to 4/3.
 3. The electrode material of claim 1, wherein v is in the range from 0 to 0.6.
 4. The electrode material of claim 1, wherein w is in the range from 0.3 to 0.77.
 5. The electrode material of claim 1, wherein said material is selected from the group consisting of: Li_(2.1)NiTi_(0.6)Nb_(0.3)O₄; Li_(2.05)NiTi_(0.8)Nb_(0.15)O₄; and Li_(2.2)NiTi_(0.2)Nb_(0.6)O₄.
 6. The electrode material of claim 1, wherein said material is formed of agglomerates of from 1 to 5 micrometers formed of particles having an average diameter in the range from 30 to 100 nanometers.
 7. A cathode having as an electronically active material comprising the material of claim
 1. 8. A lithium-ion battery containing the cathode of claim
 7. 9. A method of preparing the material of claim 1, wherein said material is formed by molten salt synthesis. 